In media handling assemblies, particularly in printing systems, accurate and reliable registration of the substrate media as it is transferred in a process direction is desirable. In particular, accurate registration of the substrate media, such as a sheet of paper, as it is delivered at a target time to an image transfer zone will improve the overall printing process. The substrate media is generally conveyed within the system in a process direction. However, often the substrate media can shift in a cross-process direction that is lateral to the process direction or even acquire and angular orientation, referred herein as “skew,” such that its opposed linear edges are no longer parallel to the process direction. Thus, there are three degrees of freedom in which the substrate media can move, which need to be controlled in order to achieve accurate delivery thereof. A slight skew, lateral misalignment or error in the arrival time of the substrate media through a critical processing phase can lead to errors, such as image and/or color registration errors relating to arrival at an image transfer zone. Also, as the substrate media is transferred between sections of the media handling assembly, the amount of registration error can increase or accumulate. A substantial skew and/or registration error can cause pushing, pulling or shearing forces to be generated, which can wrinkle, buckle or even tear the sheet.
Contemporary systems transport a sheet and deliver it at a target time to a “datum,” based on positional measurements from the sheet. That datum, also referred to herein as a delivery registration datum, can be a particular point in a transfer zone, a hand-off point to a downstream nip assembly or any other target location within the media handling assembly. Typically, the time and orientation of the sheet arriving in a sheet registration system is measured by sensors located near the input of the registration system. A controller, in the form of an automated processing device, then computes a sheet velocity command profile designed to deliver the sheet at a target time that delivery registration datum. A sheet velocity actuator commanded by the controller then executes a command profile in order to timely and accurately deliver the sheet. Examples of typical sheet registration and deskewing systems are disclosed in U.S. Pat. Nos. 5,094,442, 6,533,268, 6,575,458 and 7,422,211, commonly assigned to the assignee of record herein, namely Xerox Corporation, the disclosures of which are each incorporated herein by reference. While these systems particularly relate to printing systems, similar paper handling techniques apply to other media handling assemblies.
Such contemporary systems attempt to achieve position registration of sheets by separately varying the speeds of laterally spaced apart drive wheels in registration nip assemblies to correct for skew mispositioning of the sheet, which is also referred to as differentially driven drive or nip assemblies. As these assemblies are used to register sheets in media handling assemblies, they are also referred to as differential drive registration systems, such as that disclosed in U.S. Pat. No. 7,422,211. Separate drive motors and/or belt assemblies are often included in differential drive registration systems, for imparting an angular velocity to the driven wheels. While each motor may be connected directly to the driven wheels, belts (also referred to as timing belts) are often employed. Also, the motors may be stepper motors or DC servo motors with encoder feedback from an encoder mounted on the motor shaft, a driven wheel shaft or the idler shaft. Such registration nip assemblies also generally includes sheet sensors, which are used to detect the arrival of a sheet, its lateral position, skew and other characteristics. Temporarily driving the laterally spaced nips at slightly different rotational speeds will produce a slight difference in the total rotation or relative pitch position of each drive roll while the sheet is held in the two nips. In this way, one side of the sheet moves ahead of the other to induce a change in skew (small partial rotation) in the sheet, opposite from an initially detected sheet skew in order to eliminate and correct for the detected skew.
Sheet registration systems typically use sensors to detect a location of a sheet at various points during its transport. Sensors are often used to detect a leading edge of the sheet and/or a side of the sheet to determine the orientation of the sheet as it passes over the sensors. Based on the information retrieved from the sensors, the angular velocity of one or more nips can be modified to correct the alignment of the sheet.
FIGS. 4 and 5 illustrate a basic contemporary sheet registration system. A nip 105, 110 is formed by the squeezing together of two rolls, typically an drive roll 102 and idler roll 104, thereby creating a rotating device used to propel a sheet 125 in a process direction P by its passing between the rolls. An active nip is a nip rotated by a motor 115, 120 that can cause the nip to rotate at a variable nip velocity. Typically, a sheet registration system includes at least two active nips having separate motors. As such, by altering the angular velocities ω1, ω2 at which the two active nips are rotated, the sheet registration system may deliver the sheet 125 to the registration datum D in a registered state. A registered state meaning the sheet is delivered at a desired time with a desired positioning, orientation and rate of movement (i.e., properly register the a sheet).
Numerous sheet registration systems have been developed. For example, the sheet registration system described in U.S. Pat. No. 4,971,304 to Lofthus, which is incorporated herein by reference in its entirety, describes a system incorporating an array of sensors and two active nips. The active sheet registration system provides deskewing and registration of sheets along a process path P having an X, Y and θ coordinate system. Sheet drivers are independently controllable to selectively provide differential and non-differential driving of the sheet in accordance with the position of the sheet as sensed by the array of sensors. The sheet is driven non-differentially until the initial random skew is measured. The sheet is then driven differentially to correct the measured skew and to induce a known skew. The sheet is then driven non-differentially until a side edge is detected, whereupon the sheet is driven differentially to compensate for the known skew. Upon final deskewing, the sheet is driven non-differentially outwardly from the deskewing and registration arrangement.
A second sheet registration system is described in U.S. Pat. No. 5,678,159 to Williams et al., which is incorporated herein by reference in its entirety. U.S. Pat. No. 5,678,159 describes a deskewing and registering device for an electrophotographic printing machine. A single set of sensors determines the position and skew of a sheet in a paper process path and generates signals indicative thereof. A pair of independently driven nips forwards the sheet to a registration position in skew and at the proper time based on signals from a registration controller which interprets the position signals and generates the motor control signals. An additional set of sensors can be used at the registration position to provide feedback for updating the control signals as rolls wear or different substrates having different coefficients of friction are used.
In addition, U.S. Pat. No. 5,887,996 to Castelli et al., which is incorporated herein by reference in its entirety, describes an electrophotographic printing machine having a device for registering and deskewing a sheet along a paper process path including a single sensor located along an edge of the paper process path. The sensor is used to sense a position of a sheet in the paper path and to generate a signal indicative thereof. A pair of independently driven nips is located in the paper path for forwarding a sheet there along. A registration controller receives signals from the sensor and generates motor control drive signals for the pair of independently driven nips. The drive signals are used to deskew and register a sheet at a registration position in the paper path.
FIGS. 4 and 5 depict an exemplary sheet registration device according to the known art. The sheet registration device 100 includes two nips 105, 110 which are independently driven by corresponding motors 115, 120 for moving a sheet 125 being handled by the device 100. The motors 115, 120 are typically actuated by one or more controllers 150, which can be located almost anywhere in the system outside of the sheet path. The resulting 2-actuator device embodies a simple registration device that enables sheet registration having three degrees of freedom. The under-actuated (i.e., fewer actuators than degrees of freedom) nature makes the registration device 100 a nonholonomic and nonlinear system that cannot be controlled directly with conventional linear techniques. The control for such systems often employs open-loop (feed-forward) motion planning.
In an open-loop motion planning control process one or more sensors, such as P1, P2, E1 and E2 shown in FIG. 5, are used to determine an input position of the sheet 125 when the lead edge of the sheet is first detected by P2. An open-loop motion planner device interprets the information retrieved from the sensors as the input position and calculates a set of desired velocity profiles that will steer the sheet along a viable path to the final registered position if perfectly tracked (i.e., assuming that no slippage or other errors occur). One or more motor controllers 150 are used to control the desired velocities. The one or more motor controllers 150 generate motor voltages for the motors 115, 120. The motor voltages determine the angular velocities ω1, ω2 at which each corresponding nip 105, 110 is rotated. The sheet velocities v1, v2 at each nip 105, 110 are computed as the radius c of the drive roll 102 multiplied by the angular velocity of the roll (ω1 for 105 and ω2 for 110). The angular velocities ω1, ω2 of the nips 105, 110 transfer to the sheet in order to achieve accurate registration.
In an open-loop system, although the sheet is not monitored for path conformance during the process, an additional set of sensors, such as P3, E3 and E2 in FIG. 5, can be placed at the end of the registration system 100 to provide a snapshot of the output for adapting the motion planning algorithm. However, because path conformance is not monitored, error conditions that occur in an open-loop system may result in errors at the output that require multiple sheets to correct. In addition, although open-loop motion planning can be used to remove static (or “DC”) sources of errors, the open-loop nature of the underlying motion planning remains vulnerable to changing (or “AC”) sources of error. Accordingly, the sheet registration system may improperly register the sheet due to slippage or other errors in the system.
Accordingly, it would be desirable to provide a method and apparatus capable of more accurately registering a sheet in a media handling assembly, which overcomes the shortcoming of the prior art.